1. Field of the Invention
The present invention relates to a sheet feeder and an image forming apparatus including the sheet feeder.
2. Description of the Background Art
Image forming apparatuses such as photocopiers and printers include sheet feeders (sheet transfer device).
FIG. 49 is a longitudinal cross-sectional view illustrating a sheet feeder contained in a typical image forming apparatus. The image forming apparatuses used in business environments, such as printers and photocopiers, typically have multiple sheet feeding cassettes 21 and 22 stacked vertically to provide selectability with regard to sheet size, thickness, etc.
As can be seen in FIG. 49, the sheet feeding cassettes 21 and 22 are disposed substantially vertically and have a structure in which a sheet 91 or 92 fed from a selected sheet feeding cassette by feeding rollers 31 and 32 along guides 71 or 72 is guided diagonally upward toward a sheet feeding guide 75. The sheet feeding guide 75 is fixed almost vertically to feed sheets from each of the sheet feeder cassette 22 situated at bottom and the sheet feeder cassette 21 situated thereabove.
At the same time, in recent years, image forming apparatus such as printers and photocopiers used in business environments are required to be increasingly compact in size. This is particularly significant in the case of desktop printers. As a result, sheet feeders are formed and arranged in a complex manner in such printers.
FIG. 50 is a longitudinal cross-sectional view illustrating a typical sheet feeder. The feeding roller 31 is driven and is rotatably supported by the image forming apparatus, and an opposing roller 41 is rotatably supported by a roller base 43. The roller base 43 in turn is supported by a roller base support 51 that is fixed to the image forming apparatus. To press the opposing roller 41 against the feeding roller 31, a roller biasing member 45 such as a coil spring is inserted between the roller base 43 and the roller base support 51 to provide this biasing force, i.e., a contact pressure force, with the elastic force of the roller biasing member 45. A sheet feeding guide 75 is provided downstream of the feeding roller 31 and the opposing roller 41.
The sheet 91 fed from upstream relative to the sheet transfer direction advances into a nipping portion between the feeding roller 31 and the opposing roller 41 that nip the sheet 91 and conveys the sheet 91 downstream with a driving force provided by the rotation of the feeding roller 31. However, the sheet feeding guide 75 is bent as illustrated in FIG. 50. Consequently, depending on the arrangement of the rollers, a thick and therefore relatively stiff sheet of recording media such as a thick sheet of paper passing through the sheet feeding guide 75 may get jammed in the sheet feeding guide 75. Therefore, an upper limit is set for the thickness of the sheet in some types of image forming apparatuses. However, this is an extreme method and not a practical solution because the utility of the apparatus is compromised.
A more detailed description is now given of the mechanism of such jamming, using FIG. 49. In FIG. 49, it can be seen that the sheet 91 fed from the sheet feeding cassette 21 passes through the guide 71 and thereafter contacts the sheet feeding guide 75 with its leading end. As the thickness of the sheet 91 increases, the sheet 91 becomes stiffer. Consequently, when the leading end of the sheet 91 contacts the sheet feeding guide 75 at an angle equal to or greater than a certain angle, the contact resistance between the sheet 91 and the sheet feeding guide 75 surpasses the feeding force of the feeding roller 31, resulting in jamming or halting of the sheet 91.
To avoid this problem, Japanese patent application publication nos. H02-282131, H08-324816, 2000-072266, and 2000-118764 describe methods of decreasing the resistance of feeding sheets by changing the arrangement of the sheet feeding guide from the significantly upright position to a more horizontal position, to form a smaller, more acute contact angle between the leading end of a sheet and a sheet feeding guide. However, desktop printers for use in small offices are required to be compact in size in terms of space-saving. Against this demand, the size of the apparatus increases by the methods described above. That is, there is a trade-off between the improvement of feeding performance and size reduction of the apparatus, as illustrated in FIG. 50.